Changes to Lucene are only persisted to disk during a Lucene commit, which is a
relatively expensive operation and so cannot be performed after every index or
delete operation. Changes that happen after one commit and before another will
be removed from the index by Lucene in the event of process exit or hardware
failure.

Because Lucene commits are too expensive to perform on every individual change,
each shard copy also has a transaction log known as its translog associated
with it. All index and delete operations are written to the translog after
being processed by the internal Lucene index but before they are acknowledged.
In the event of a crash, recent transactions that have been acknowledged but
not yet included in the last Lucene commit can instead be recovered from the
translog when the shard recovers.

An Elasticsearch flush is the process of performing a Lucene commit and
starting a new translog. Flushes are performed automatically in the background
in order to make sure the translog doesn’t grow too large, which would make
replaying its operations take a considerable amount of time during recovery.
The ability to perform a flush manually is also exposed through an API,
although this is rarely needed.

The data in the translog is only persisted to disk when the translog is
fsynced and committed. In the event of hardware failure, any data written
since the previous translog commit will be lost.

By default, Elasticsearch fsyncs and commits the translog every 5 seconds
if index.translog.durability is set to async or if set to request
(default) at the end of every index, delete,
update, or bulk request. More precisely, if set
to request, Elasticsearch will only report success of an index, delete,
update, or bulk request to the client after the translog has been successfully
fsynced and committed on the primary and on every allocated replica.

How often the translog is fsynced to disk and committed, regardless of
write operations. Defaults to 5s. Values less than 100ms are not allowed.

index.translog.durability

Whether or not to fsync and commit the translog after every index, delete,
update, or bulk request. This setting accepts the following parameters:

request

(default) fsync and commit after every request. In the event
of hardware failure, all acknowledged writes will already have been
committed to disk.

async

fsync and commit in the background every sync_interval. In
the event of hardware failure, all acknowledged writes since the last
automatic commit will be discarded.

index.translog.flush_threshold_size

The translog stores all operations that are not yet safely persisted in Lucene
(i.e., are not part of a Lucene commit point). Although these operations are
available for reads, they will need to be reindexed if the shard was to
shutdown and has to be recovered. This settings controls the maximum total size
of these operations, to prevent recoveries from taking too long. Once the
maximum size has been reached a flush will happen, generating a new Lucene
commit point. Defaults to 512mb.

index.translog.retention.size

The total size of translog files to keep. Keeping more translog files increases
the chance of performing an operation based sync when recovering replicas. If
the translog files are not sufficient, replica recovery will fall back to a
file based sync. Defaults to 512mb

index.translog.retention.age

The maximum duration for which translog files will be kept. Defaults to 12h.